1. Field of the Invention
The present invention relates generally to measurement of distance to visual landmarks for applications including navigation and mapping, and more particularly navigation and mapping in a Global Positioning System (GPS) denied environment.
2. Description of the Related Art
There is a growing need to provide underground, urban, and indoor navigation without the use of external augmentation aids or GPS. Tracking or navigation systems often make use of measurements from sensors to aid in determining a location (“localization”) or an orientation (attitude and heading) or a pose (position and orientation) of an object such as a person, a vehicle or a robot as it navigates in an environment, such as within the bounds of a building. A variety of types of sensors are available for such systems, including sensors that measure a relative location between a sensor and a target. An example of such a sensor/target combination is a pulsed laser emitter and a photosensor that can determine a distance traveled by the laser pulse by measuring time of flight. Different types of sensors measure different aspects of the relative pose of a sensor and a target, such as a range, direction, or relative orientation.
Location and mapping is a fundamental problem in mobile robotics. While a robot navigates in an unknown environment, it must incrementally build a map of its surroundings and, at the same time, localize itself within that map. One popular solution is to use Simultaneous Localization and Mapping (SLAM) algorithms, treat localization and mapping as an estimation problem and apply a Kalman filter.
As will be disclosed below, the present invention supplies technology components that can supply target range and bearing information to a SLAM algorithm using a very compact, low-power sensor. Further, the disclosed design allows the range of the sensor to be extended at the expense of spatial resolution in cases where the range from sensor to target is too great for the sensor to operate normally.
Flash LIDAR, which illuminates a scene using a single laser pulse that is optically spread to cover the field of view of a gated focal plane array, outputs an image in which each pixel is labeled with both brightness and range. This ability to simultaneously range to all objects in a scene is extremely useful for unmanned ground vehicles as well as for general reconnaissance and surveillance.
The alternative to Flash LIDAR is a scanning LIDAR. This uses a pencil laser beam with a single sensor and a moving mirror assembly that scans the beam across the scene. Aside from the potential difficulties with rapidly moving parts, the fact that all pixels in a scene are sampled sequentially can lead to distortion or misperception if there is relative motion between the sensor and all or part of the scene.
A major drawback to the use of flash LIDAR is that the spreading of the illuminating laser beam introduces a quadratic fall-off in illumination with range. In a GPS-Denied Navigation activity for dismounted troops, the range of the flash LIDAR is limited due to the requirement that the LIDAR be body-worn. Both available battery power and heat that must be dissipated from the unit contribute to this restriction. Heretofore, it has been necessary to sacrifice field of view by using longer focal length optics on the flash LIDAR in order to achieve acceptable range for a man-worn system.
U.S. Pat. No. 6,414,746, issued to R. Stettner, et al., entitled “3-D Imaging Multiple Target Laser Radar,” assigned to Advanced Scientific Concepts, Inc., Santa Barbara, Calif., discloses a device which uses a single pulse from a pulsed light source to detect objects which are obscured by camouflage, fog or smoke but otherwise enveloped by a light-transmitting medium. The device simultaneously operates in two modes, light reflected from the nearest object is processed to form a three-dimensional image by an array of pixels. This first image is based upon the light-pulse transit time recorded in each pixel. Each pixel also contains a high-speed analog memory that sequentially stores reflected signals at a repeated time interval. The first reflection acts as a time base that controls when the analog memory begins or ends the storage sequence. The first return could be from a camouflage net and the amplitudes of the return signals, after the first return, would then be from objects behind the net. Computer processing these amplitudes reveals the three-dimensional nature of the obscured objects. The device consists of the pulsed light source, optics for collecting the reflected light, a sensor for detecting the light and converting it to electrical data, drive and output electronics for timing and signal conditioning of data generated by the sensors and a computer for processing the sensor data and converting it to a three dimensional image. The sensor collects and processes the light data in a unique manner, first converting it to electricity by a number of alternate detector technologies and then using integrated circuit chips which consist of a two dimensional array of electronic pixels also called unit cells. The two dimensional array defines two dimensions of the image. Stored within each unit cells is data associated with the third dimension, ranges of targets, and amplitudes of target reflections. This data are read out of the integrated circuit chip in the time interval between laser pulses to a processing computer. The processing computer corrects the data and, by means of computer algorithms specific to the device, converts the data to a three-dimensional image of one or more targets. This image may be viewed or processed electronically to isolate targets.
Other Advanced Scientific Concepts, Inc. assigned patents include U.S. Pat. Nos. 6,133,989, 5,696,577, and 5,446,529.
None of the aforementioned references provide an efficient flash LIDAR system for laser range finding that is particularly suitable for man worn applications.